The present invention relates to a device and method for controlling the machining depth of a machine for machining multilayer printed circuit boards.
In the fabrication of multilayer printed circuit boards, the board must be drilled and/or milled to a predetermined, precisely controlled depth, the purpose of the machining operation, in fact, normally being to reach a given conducting layer and no further. Owing to the uneven upper surface of the board, however, controlling the machining depth with reference to the machine itself, e.g. the worktable, would inevitably result in depth control errors.
In one known machine for drilling multilayer boards, machining depth is controlled according to the position of the tip of the tool on the spindle with respect to the board holder bush, which therefore acts as a feeler for the upper surface of the board, from which the machining depth is controlled. Such a machine, however, presents the drawback of the position of the tip of the tool with respect to the board holder bush varying as a result of both wear and the temperature of the spindle.
In another known drilling machine, the reference position of the tip of the tool is determined by bringing the spindle up to the steady-state temperature and probing with the tool a reference board fitted to the edge of the worktable. More specifically, the tool locates an air-cushion piston fitted to the reference board; and an electric reference signal is generated by a proximity sensor as the tool contacts the piston.
This device presents the drawback of involving a small amount of movement of the piston to generate the reference signal. Moreover, the assembly comprising the air-cushion piston and proximity sensor is fairly complicated and expensive to produce.